Organic SpectroscopyWorkbook

Organic SpectroscopyWorkbookTOM FORREST

JEAN-PIERRE RABINE

MICHEL ROUILLARD

A John Wiley & Sons, Ltd., Publication

This edition first published 2011 2011 John Wiley and Sons Ltd.

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Contents

Preface vii

Acknowledgements ix

How to Use this Workbook 1

Preliminary Observations 2

Centres of Unsaturation 8

Exercises 1–100 10

Infrared Data 210IR Frequencies: Display of Infrared Database by Frequency 210IR Groups: Display of Infrared Database by Functional Group 216

1H NMR Data 221General Tables of Functional Groups 221Comparison of δ of Alkyl Groups Bearing a Substituent in α or β Position 225Alkyl Groups with an α Substituent 226Alkyl Groups with a β Substituent 226

Tables of δ for −CH2− Doubly Substituted in α Position 227Tables of δ for Aromatics 229Coupling Constants 230

13C NMR Data 233General Ranges for CH3−, −CH2−, >CH−. . . 233Chemical Shift Ranges for Functional Groups 234Chemical Shift Estimations 234

Mass Spectrometry Data 240Main Fragments Observed 240Atomic Masses 242Isotopic Abundances in % 243

Internet Resources 254

Glossary of Terms Used in the Exercises 255

Answers 257

Index 261

Preface

This workbook arose out of a long standing project for the development of computer basedprograms to assist students in learning how to deduce the structures of organic compoundsfrom spectroscopic data. The ability to elucidate chemical structures is built largely onimplicit heuristics and informal experience accumulated by solving many examples. Theaccumulation of this experience is greatly facilitated by these computer based programs,http://spectros.unice.fr/. However, the knowledge that is so arduously obtained at the com-puter may evaporate rapidly unless some reinforcement mechanism is in place.

This workbook was originally conceived as a way to provide the computer users with avehicle for making a lasting record of their learning experience. However, we recognizedthat a similar situation occurs for students who prefer to undertake their study without usingcomputer-based exercises. They could also benefit from a workbook where they could getassistance in solving spectral problems and have a record that could be used for review andreinforcement. The resulting workbook was designed so that it can be used with or withoutthe computer based program. The computer exercises, which are freely available online toany user, provide more detailed explanations and supplementary graphical material for thestructural determinations.

For each problem in the workbook we provide four spectra together with a detailedanalysis of the spectra and an explanation of the derivation of the structure of the compound.

The level of detail provided in the solution decreases gradually, until near the end of theset of exercises, very little or no analysis is given. The solutions to all of the exercisesare provided, but no structural formulae are shown on the work area of the exercise, sothat the student may follow along the analysis and draw the structural components as theyare revealed.

There are many advanced techniques available in modern spectrometers that may beemployed to assist in the solution of more difficult structural problems. In these exerciseswe have chosen to give low resolution electron impact mass spectra because these spectra arereadily available, and are rich in structural information. Modern mass spectrometry employsa multitude of other ionization techniques that can be used in particular circumstances toprovide specific information, but these are beyond the scope of an introductory spectralexercise book. In the field of NMR spectroscopy there are numerous sophisticated pulsesequences available that can generate various multidimensional spectra which are used formore difficult structural problems. These techniques are particularly useful for large complexstructures, but they are aids that still require the user to have an understanding of thefundamentals of spectroscopic interpretation, which is the goal of these exercises.

We hope that students will find that using this workbook will prove to be a pleasant andrewarding experience.

Acknowledgements

We have used the resources of our university libraries extensively in preparing this workbook,but have found that we made even more extensive use of the resources available on theinternet. We are indebted to the many people who have taken the time to place their data atthe disposal of the scientific community in this manner. This easy access to spectral data hasbeen of immense assistance to us in the preparation of this workbook. The availability ofsuch a large amount of primary data allowed us to find very good model compounds whichgave more specific support for the assignments than possible from the general correlationtables of secondary publications.

It is impossible to list all of the internet sites that we visited during the preparation of themanuscript; however, we would like to specifically acknowledge the providers of those sitesthat we have found we were consistently using. These are for the most part sites providedby institutions, universities and chemical suppliers:

NIST, National Institute of Standards and Technology, Chemistry Webbook;National Institute of Advanced Industrial Science and Technology (AIST, Japan), Spectral

Database for Organic Compounds;

Sigma-Aldrich Corporation, NMR and IR spectra;Acros Organics, IR spectra;University of Wisconsin, Chemistry Department, H. J. Reich, compilation of NMR chemical

shifts and coupling constants.

We are grateful for the support of:

Universite de Nice Sophia-Antipolis;Universite Numerique Thematique:UNISCIEL (Universite des Sciences en Ligne)Dalhousie University

How to Use this Workbook

The exercises give spectral charts and verbal analyses of the spectra, but the structuraldiagrams that would complete the picture are missing. Although annotated structureswould be very helpful in the spectral assignments, they are deliberately omitted to avoid apremature revelation of the answer. You don’t want to see the solution before you have hadthe opportunity to think about the problem. You may want to use a small sheet of paper tocover the conclusion/summary section as you try to solve each structure. There is adequatewhite space available on the pages to allow you to draw the appropriate structures, and tomake notes on the spectral assignments. Complete structural drawings are available in theAnswers section of the workbook. Each exercise has a second number in brackets whichidentifies the number by which the compound can be located in the Answers section, andin the computer-based exercises. The structures are not presented in the same order asthe exercises, thus lessening the possibility of an unwanted exposure of the answer to thenext exercise.

There is no single method that provides the best way to solve these problems. There isusually an abundance of information that allows many paths to the solution. However, itis unlikely that the complete structure will be revealed intact, so the general approach isto derive the partial structures of the molecule and to connect them to form the completestructure. The order of the spectral analysis provided in each exercise is not presented as thebest way to approach the problem. Many people do a preliminary scan of the spectra to lookfor significant features, identify the responsible structural component, then verify its presenceusing other spectra. For example, a carbonyl group that stands out in the IR spectrum canbe verified by the peak at the appropriate chemical shift in the 13C NMR spectrum. As onemoves from spectrum to spectrum like this, it is imperative that an organized approach bemaintained and that good notes are preserved.

It is recommended that you write down the partial structures as you derive them. Youcan always erase them if you make a mistake or, better still, simply draw a mark through

Organic Spectroscopy Workbook, First Edition. Tom Forrest, Jean-Pierre Rabine, and Michel Rouillard. 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.

them, and make notes to ensure that you have them for review and can learn from yourmistakes. Take the opportunity to make detailed notes before you leave each exercise. Asclear as it will be in your mind at that moment, it is amazing how the details evaporatewith time.

All the 13C NMR DEPT spectra are DEPT-135 spectra in which methylene carbons showa negative signal and quaternary carbons show no signal. Methyl and methine carbons showup as normal positive signals.

All the mass spectral charts are low-resolution electron impact mass spectra (EIMS). Thesespectra frequently provide the molecular weight of the compound, and the high degree offragmentation also offers valuable information to those with knowledge of reactivity oforganic compounds. In some examples, the results of high-resolution mass spectra (HRMS)are provided which can be used to obtain the molecular formula directly.

The exercises are organized somewhat in the order of complexity of the molecule andlevel of explanation provided. You will find very simple spectra and detailed explanations atthe beginning, and eventually more complex exercises with less in the way of explanation.For the last 20 exercises, only general hints and suggestions are provided. In these exercisesthe 1H NMR spectral data is provided in standard text format to show the coupling constantswhere appropriate. These data should be compared to the spectral pattern, so that you couldwrite such a verbal description of the spectrum.

The resource material you require to analyse the spectra is provided in tables andalgorithms in separate data sections for each of the spectroscopy types.

The exercises presented in this work book are also available in an on-line program,Multispectroscopy, which you may access at spectros.unice.fr. This program provides moredetailed explanations, as well as elaborate diagrams and graphics, to help to explain thespectral analysis and structure determination. You may wish to take advantage of thisadditional resource, while you are using this workbook.

Preliminary Observations

It is easy to get some information from a quick glance at some spectra. There are sometypical shapes that give a good indication of particular structural features. It is essential thatthese deductions are verified by detailed inspection of the spectrum, and with data fromother spectra.

Infrared

This indicates an alcohol.

This indicates an acid.

Organic Spectroscopy Workbook, First Edition. Tom Forrest, Jean-Pierre Rabine, and Michel Rouillard. 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.

This indicates an alkene.

This indicates a terminalalkyne.